in my reaserch i found that mass moment of inertia is principle parameter in model with mass IIrregularity, so i try figure out how modal analysis run in etabs and opensees and what difference between them. i trying almost everythings to verify it but Unfortunately all my work was unsuccsesfull. i builded the model in etabs and can verify my model with 0.33% error, but i couldn't verify model in opensees. i use a model from a reasercher's article, he note that the modal frequences of first three mode is 1.16, 1.96, 2.37 HZ respectively. the model has no beam, so i used rigid diaphragm to equal dof of top nodes of columns. I'm trying to model a 3D one story buiding with mass IIrregularity, the model has 4 elastic columns and rigid diaphragm. So, how should i model strut so that it will be tension free and it takes only compression? how can I model it in ETABS so that it will not take any live and dead load from the structure? and i am not quit clear about that should strut is modeled for Nonlinear analysis( Pushover, nonlinear time history ) only OR can it be modeled for Linear dynamic such (RESPONSE SPECTRUM, LINEAR TIME HISTORY ) analysis? and how can i model it in Linear dynamic analysis? And i am not able to make it not to take Dead and live load response,as infill should not take any dead and live load from the structure. BUT i am unable making it not to take Tension in strut when running the analysis. I have modeled strut assuming pinned jointed frame element having material properties as per masonry property as per Indian Standard. I have to consider infill consideration in the building, So i have calculated Strut width as per FEMA 356, using masonry property of Indian Standard. I am doing Response spectrum and Linear time history analysis in RC building for my project.

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